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          <h1 class="post-title" itemprop="name headline">Java中的静态代码块</h1>
        

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        <ul>
<li><p>java编译处理静态变量的顺序，比如 <strong>public static int i =10</strong>这行代码实际上是两个过程，先声明变量，在static中进行初始化。如果没有static块，编译器会默认提供一个static块。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Static</span></span>&#123;</span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">int</span> i = <span class="number">10</span>;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Static</span></span>&#123;</span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">int</span> i;</span><br><span class="line">    <span class="keyword">static</span>&#123;</span><br><span class="line">        i = <span class="number">10</span>;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line">上面两段的代码的字节码是一模一样的，字节码如下：</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">base</span>.<span class="title">Static</span> </span>&#123;</span><br><span class="line">  <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">int</span> i;           <span class="comment">//声明静态变量</span></span><br><span class="line"></span><br><span class="line">  <span class="keyword">public</span> base.Static();</span><br><span class="line">    Code:</span><br><span class="line">       <span class="number">0</span>: aload_0</span><br><span class="line">       1: invokespecial #1                  // Method java/lang/Object."&lt;init&gt;":()V</span><br><span class="line">       <span class="number">4</span>: <span class="keyword">return</span></span><br><span class="line"></span><br><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(java.lang.String[])</span></span>;</span><br><span class="line">    Code:</span><br><span class="line">       <span class="number">0</span>: <span class="keyword">return</span></span><br><span class="line"></span><br><span class="line">  <span class="keyword">static</span> &#123;&#125;;</span><br><span class="line">    Code:</span><br><span class="line">       <span class="number">0</span>: bipush        <span class="number">10</span>          在静态代码块内给把<span class="number">10</span>压入栈</span><br><span class="line">       2: putstatic     #2                  // Field i:I</span><br><span class="line">       <span class="number">5</span>: <span class="keyword">return</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
</li>
<li><p>编译器会对多个static块进行优化。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Static</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">int</span> i;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">static</span> &#123;</span><br><span class="line">        i = <span class="number">10</span>;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">static</span> &#123;</span><br><span class="line">        i = <span class="number">20</span>;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line">字节码如下：</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">base</span>.<span class="title">Static</span> </span>&#123;</span><br><span class="line">  <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">int</span> i;</span><br><span class="line"></span><br><span class="line">  <span class="keyword">public</span> base.Static();</span><br><span class="line">    Code:</span><br><span class="line">       <span class="number">0</span>: aload_0</span><br><span class="line">       1: invokespecial #1                  // Method java/lang/Object."&lt;init&gt;":()V</span><br><span class="line">       <span class="number">4</span>: <span class="keyword">return</span></span><br><span class="line"></span><br><span class="line">  <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(java.lang.String[])</span></span>;</span><br><span class="line">    Code:</span><br><span class="line">       <span class="number">0</span>: <span class="keyword">return</span></span><br><span class="line"></span><br><span class="line">  <span class="keyword">static</span> &#123;&#125;;</span><br><span class="line">    Code:</span><br><span class="line">       <span class="number">0</span>: bipush        <span class="number">10</span></span><br><span class="line">       2: putstatic     #2                  // Field i:I</span><br><span class="line">       <span class="number">5</span>: bipush        <span class="number">20</span></span><br><span class="line">       7: putstatic     #2                  // Field i:I</span><br><span class="line">      <span class="number">10</span>: <span class="keyword">return</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
</li>
</ul>
<p>从上面的字节码看出，编译器会将多个static块按照代码的前后位置重新组合成一个static块。<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"> <span class="keyword">static</span> &#123;</span><br><span class="line">        i = <span class="number">10</span>;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">int</span> i = <span class="number">100</span>;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">static</span> &#123;</span><br><span class="line">        i = <span class="number">20</span>;</span><br><span class="line">    &#125;</span><br><span class="line">对应的字节码应该为：</span><br><span class="line"><span class="keyword">static</span> &#123;&#125;;</span><br><span class="line">    Code:</span><br><span class="line">       <span class="number">0</span>: bipush        <span class="number">10</span></span><br><span class="line">       2: putstatic     #2                  // Field i:I</span><br><span class="line">       <span class="number">5</span>: bipush        <span class="number">100</span></span><br><span class="line">       7: putstatic     #2                  // Field i:I</span><br><span class="line">      <span class="number">10</span>: bipush        <span class="number">20</span></span><br><span class="line">      12: putstatic     #2                  // Field i:I</span><br><span class="line">      <span class="number">15</span>: <span class="keyword">return</span></span><br></pre></td></tr></table></figure></p>

      
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